1. Field of the Invention
The present invention relates to asphalt roofing shingles, and protective granules for such shingles, and processes for making such granules and shingles.
2. Brief Description of the Prior Art
Pigment-coated mineral rocks are commonly used as color granules in roofing applications to provide aesthetic as well as protective functions to the asphalt shingles. Roofing granules are generally used in asphalt shingle or in roofing membranes to protect asphalt from harmful ultraviolet radiation.
Roofing granules typically comprise crushed and screened mineral materials, which are subsequently coated with a binder containing one or more coloring pigments, such as suitable metal oxides. The binder can be a soluble alkaline silicate that is subsequently insolubilized by heat or by chemical reaction, such as by reaction between an acidic material and the alkaline silicate, resulting in an insoluble colored coating on the mineral particles. Preparation of colored, coated roofing granules is disclosed for example, in U.S. Pat. No. 2,981,636 of Lodge et al. The granules are then employed to provide a protective layer on asphaltic roofing materials such as shingles, and to add aesthetic values to a roof.
U.S. Pat. No. 2,732,311 of Hartwright discloses a method for preparing roofing granules having metal flakes, such as aluminum flakes, adhered to their surfaces, to provide a radiation-reflective surface. Rock granules are first mixed with kaolin clay, and then a stream of sodium silicate solution is added. A tacky viscous film is developed on the surface of the granules by passing air through the mixture while it is being mixed, and a fine grade of metal flakes is added, and the flake-coated granules are subsequently fired to cure the clay-silicate binder.
Pigments for roofing granules have usually been selected to provide shingles having an attractive appearance, with little thought to the thermal stresses encountered on shingled roofs. However, depending on location and climate, shingled roofs can experience very challenging environmental conditions, which tend to reduce the effective service life of such roofs. One significant environmental stress is the elevated temperature experienced by roofing shingles under sunny, summer conditions, especially roofing shingles coated with dark colored roofing granules. Although such roofs can be coated with solar reflective paint or coating material, such as a composition containing a significant amount of titanium dioxide pigment, in order to reduce such thermal stresses, this utilitarian approach will often prove to be aesthetically undesirable, especially for residential roofs.
Asphalt shingles coated with conventional roofing granules are known to have low solar heat reflectance, and hence will absorb solar heat especially through the near infrared range (700 nm-2500 nm) of the solar spectrum. This phenomenon is increased as the granules covering the surface become dark in color. For example, while white-colored asphalt shingles can have solar reflectance in the range of 25-35%, dark-colored asphalt shingles can only have solar reflectance of 5-15%. Furthermore, except in the white or very light colors, there is typically only a very small amount of pigment in the conventional granule's color coating that reflects solar radiation well. As a result, it is common to measure temperatures as high as 77 degrees C. on the surface of black roofing shingles on a sunny day with 21 degrees C. ambient temperature. Absorption of solar heat may result in elevated temperatures at the shingle's surroundings, which can contribute to the so-called heat-island effects and increase the cooling load to its surroundings.
There is a continuing need for roofing materials, and especially asphalt shingles, that have improved resistance to thermal stresses while providing an attractive appearance. In particular, there is a need for roofing granules that provide increased solar heat reflectance to reduce the solar absorption of the shingle, while providing a wide range of colors including deep-tone colors to maintain the aesthetic value of the system.